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This error usually means that on your website's server the session folder is not writable or doesn't exists, probably isn't even set. This is about the 'session.save_path' setting in the php.ini of the server. Create a test file (let's call it info.php) with this code:
<?php $path = ini_get('session.save_path'); echo 'session.save_path: ' . '<br>'; var_dump($path); echo '<br>is_writable: '; var_dump(is_writable($path));
then upload it into your FTP and take a look at it in your browser: http://example.com/info.php
You will probably see something similar:
session.save_path: string(0) "" is_writable: bool(false)
Which means, that the session.save_path is not set. You should contact your server host to help you resolve the problem!
If you see something different, the result should be that the session.save_path has to be a path, like "/this/is/a/path" and the is_writable has to be "true". If it's not what you see, contact the server host.